Research On Hybrid Combustion Control Strategy Based On Interaction Of Mixing And Chemical Factors

Due to the shortage of energy and noble metal resources and the deterioration ofenvironment, high thermal efficiency and clean combustion in diesel engines becomesmore and more important and low temperature combustion including HomogeneousCharge Compression Ignition (HCCI) combustion and EGR-diluted combustion hasattracted more and more attention in recent years. Low-temperature combustion underlow load has been achieved through various means by researchers at home and abroad.But so far, the choke points of load expansion based on low temperature combustionhave not been effectively resolved, mainly due to poor thermal efficiency, soot, HCand CO emissions when using high EGR rate. Therefore, achieving low temperaturecombustion with high efficiency and low emissions at higher load becomes the core ofthe study. In this dissertation, for the purpose of achieving high thermal efficiency andlow NOx, soot emissions of the whole working conditions, complicated physical andchemical processes involved in the new concept combustion are further studied byengine experiment, CFD numerical simulation and PLIF method.Experimental results of traditional low-temperature combustion show that usinga large EGR rate can achieve very low NOx and soot emissions, but this result isbased on the penalty for deterioration of CO, HC and indicated thermal efficiency.Increasing boost pressure moderately with low and middle EGR rate can get bettertrade-off NOx and soot, but when using a large EGR rate, it is difficult tofundamentally solve the problem of low thermal efficiency with higher boostpressure.Experimental results of diesel-fuelled PCCI show that, based on single earlyinjection strategy, ultra low NOx and soot emissions and high indicated thermalefficiency in low load conditions can be achieved by optimization of EGR rate andinjection timing, but due to poor mixing and local rich areas increasing, knock andpoor soot, HC, CO and thermal efficiency will not be avoided when expanding itsload. In comparison, the PCCI combustion based on multi-pulse injections has a hugeadvantage on load extension. The knock is reduced due to the fuel stratification andNOx and soot are all reduced due to reducing local rich areas both by multi-pulseinjections. By collaborative optimization of RIVCT, boost pressure, EGR rate andother parameters based on multi-pulse injections, very low emissions can be achievedin the middle load (IMEPg of1.1MPa) and at this time with indicated thermal efficiency of51%.A new concept of high density-low temperature combustion is put forward bytheoretical analysis, optical spray test, CFD numerical simulation and engine test. Theresults show that charge volume heat capacity and mixing rate, both controlled bycharge density, can affect NOx emissions. Higher charge density and increased EGRrate have the similar effects on reducing combustion temperature and increasingcharge density can compensate missing oxygen content for the introduction of thelarge number of EGR which effectively improves the soot emissions and thermalefficiency. By means of RIVCT the charge density is maximized within the cylinderpressure limitation and very low emissions and higher indicated thermal efficiency areachieved at full load conditions (IMEPg of2.2MPa)Based on the research above, a hybrid combustion control strategy is put forwardbase on interaction of mixing and chemical factors. For the hybrid combustion controlstrategy strategy, different control strategies should be used at different loadconditions for a high BMEP heavy-duty diesel engine: at low load, PCCI combustionbased on single early injection and high EGR rate should be used; at middle load,PCCI combustion based on the technologies of multi-pulse injections, high boostpressure and RIVCT should be used; at high even full loads, high density-lowtemperature combustion should be used. The results show that hybrid combustioncontrol strategy will achieve very low emissions with high thermal efficiency in a HDdiesel engine while canceling or reducing dependence on complicated after treatment.